GEOLOGIC INFLUENCES ON NEAR-SURFACE PROCESSES CONTROLLING INFILTRATION, RUNOFF, AND RECHARGE IN MOUNTAINOUS TERRAIN

The quantity of recharge that reaches the water table in mountainous terrain is primarily governed by surficial and near-surface processes of rainfall, snow accumulation, snow melt, evapotranspiration, infiltration, and runoff. Physical properties of the soil, such as soil water storage and infiltration capacity, along with evapotranspiration, dictate whether percolating water will reach the soil-bedrock interface. Once there, the water will either infiltrate into fractures or porous rock, or pond, where it can be removed by evapotranspiration, move down dip to infiltrate, or become runoff to nearby stream channels. As such, the regional near-surface geologic materials and the structure and permeability of the underlying bedrock control the quantity of direct infiltration and the generation of runoff, thereby influencing whether basins are likely to be dominated by runoff and result in downstream recharge, or to receive recharge directly into bedrock. For example, high permeability limestone reduces the potential for large quantities of runoff; whereas low permeability granites allow little infiltration into the bedrock thus creating greater potential for runoff. Large-scale geologic features and properties, such as faults and fractures, sloping strata, matrix permeability, and degree of weathering, all influence the deeper movement of water through the unsaturated zone that is available following near-surface hydrologic processes, and can also be key factors in regional hydrologic processes, water availability and quality, and timing of recharge to the regional water table. Understanding mountain recharge on both a local and regional basis, therefore, requires an assessment of the distribution of the surface structure and the hydrologic properties of near-surface geology and the geologic strata in the unsaturated zone in mountainous terrain.